This invention relates to herbicidal compositions for controlling undesirable brush in rangelands and woodlands and, more particularly, to herbicidal compositions which are dry-compacted into swellable pellets having improved properties for aerial application and use in low-rainfall areas.
Undesirable weeds and thorny brush growth such as mesquite, huisache, yaupon, McCartney rose and the like infest millions of acres of rangeland. Once heavily infested, such rangelands are no longer suited for livestock grazing.
Broad-spectrum herbicides, which are normally applied to infested acreage with aerial or ground sprays, or by a dry granule broadcast method, destroy not only the undesirable brush, but also much needed grasses and other ground cover. Although selective herbicides can be used to avoid injury to ground cover, the high concentrations of active ingredient which are needed to destroy the hardy brush varieties can make the cost of selective herbicides prohibitive.
In woodlands, particularly pine woodlands which are "farmed" to produce trees for the pulp and paper industry, brush growth can retard the development of young trees and may increase the planting-to-harvesting cycle by as much as eight years. Broadcast applications of conventional herbicide formulations at rates high enough to control the undesirable brush can kill or severely damage the young trees.
The compositions of this invention are aimed typically at woodland and brush areas which are arid and have low rainfall. Any herbicide which is to be useful for this type of area must require very little water to make the material active in the soil. Furthermore, economy of application, where large areas and difficult terrain are involved, favors the use of airplanes.
With certain herbicides--such as the ones used in the compositions of this invention--selection between the large brush that must be controlled vs. the seedling trees or grass for grazing whose growth the treatment should promote, can be obtained by applying the herbicide as large pellets or briquettes on a grid pattern, for example on a 1-3 meter grid. As the herbicide is moving downward from the relatively few loci of the grid, the large root systems of the brush can pick up a lethal dose of herbicide while in the remaining areas the grass or the seedling trees grow without damage.
This combination of application requirements can only be met with large pellets or balls which have very special physical properties. The handling, shipment, storage and use of such material requires that these pellets be hard, crush and impact resistant, and unaffected by high relative humidity, while, at the same time, after application, the pellets must swell and disintegrate after only small amounts of rainfall to release the active herbicide.
Thus, there is need for a composition having a critical set of properties which are in part contradictory and are difficult to attain. Furthermore, it is important to produce these compositions, i.e. these pellets and balls, economically. Hence, expensive wet extrusion and drying processes should be avoided in favor of dry-compaction or briquetting.